The researchers followed 25,892 men ages 30 to 87 who took treadmill tests to determine the most exercise they could do. The men were followed for an average of 10 years. In this time, there were 133 deaths from cancers related to smoking and 202 deaths from other cancers.
The most-fit men had a 55 percent lower risk of all-cause cancer death than did low-fit men, and moderately fit men had a 38 percent lower risk, said researchers Chong Do Lee of West Texas A&M University and Steven N. Blair of the Cooper Institute of Dallas.
To be moderately fit, a person would have to run 20 to 40 minutes, three to five times a week. To be most fit, a person would have to be at the recreationally competitive level.
The most-fit men had a 46 percent lower risk, and moderately fit men had a 34 percent lower risk of cancers unrelated to smoking. These diseases include cancers of the colon and prostate, and leukemia, which affects white blood cells.
The most-fit men had a 66 percent lower risk and the moderately fit men a 43 percent lower risk of cancers related to smoking, such as cancers of the lung and mouth. If the least-fit smokers had become fit, they would have reduced their death risk by 13 percent, a statistical analysis in the study concluded.
The most-fit men were least likely to smoke, but almost 10 percent of them did, as did 20 percent of moderately fit men and 33 percent of sedentary men. The most-fit and moderately fit smokers were still less likely to die than were the sedentary smokers, Lee said. "If you are a high-fit smoker, then you should have a lower risk," Lee said.
Just the same, Lee and other experts say exercise is no substitute for giving up smoking. The study found that nonsmokers who were the most physically fit had the lowest risk of dying of cancer.
The researchers theorized that the heavy breathing that comes with vigorous activity clears the lungs of some cancer-causing chemicals associated with smoking. And fitness may help the body in other ways, such as improving defensive systems that may keep tumors from forming, they said.
"This is a seminal study," said Scott Leischow, chief of the tobacco control research branch of the National Cancer Institute. He was not involved in the study.
The study should encourage smokers who can't -- or won't -- quit to be at least good exercisers, he said.
Smokers should check with a doctor before starting an exercise program, because smoking can cause otherwise hidden problems such as heart disease, which the stress of exercise may trigger, Leischow said.
